Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires a gas ion bombardment of the target having a face formed of a desired material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials traverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
In conventional target cathode assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in well-known dispositions in order to form the above-noted magnetic field in the form of a loop or tunnel extending around the exposed face of the target.
In order to achieve good thermal and electrical contact between the target and the backing plate, these members are commonly attached to each other by use of soldering, brazing, diffusion bonding, clamping, epoxy cements, or with interlocking annular members.
The use of indium tin oxide coatings has increased due to their demand in conjunction with the production of conductive transparent layers on paper, or glass and transparent conductivity layers in liquid crystal devices.
These materials are popularly coated via cathodic sputtering techniques wherein an indium tin oxide sputter target and copper backing plate assembly are employed in the process. Typically, in these target and backing plate assemblies, the target is bonded to the backing plate by means of an indium tin solder which may have a melting point of about 150.degree.-160.degree. C.
Heating of the target and backing plates to such temperatures to melt the solder is problematic in that the brittle indium tin oxide target may crack and the copper backing plate may warp or shrink. The problem is exacerbated by the current trend toward use of increasingly larger size target/backing plate assemblies.
Similar problems exist in the bonding of titanium targets to corresponding aluminum or copper backing plate members. The use of Ti and Ti alloy layers is increasing in semiconductor applications and accordingly, there is an increased demand for Ti and Ti alloy sputter targets.
Conventionally, these Ti target and Cu backing plate materials are solder bonded by Sn and Ag solders which have liquidus temperatures in excess of about 200.degree. C. When the Ti or Ti alloy target/Al or Cu backing plate members are so heated, the large difference in thermal expansion properties between the two leads to bonding problems caused by part warpage and differential part contraction.
Diffusion bonding of Ti target/Al backing plate assemblies has proven very successful. However, with the advent of extra large target/backing plate assemblies as called for by current commercial demand, the purchase of large scale diffusion bonding presses poses a significant capital expenditure item.
In another sputter target development, tubular tin indium oxide targets have been provided to sputter coat large display panels. These targets are mounted over a coaxial, tubular backing member. In the past, it has been difficult to bond the target to the backing member along the annular target-backing member interface due to the curvature of the interfacial surfaces. Additionally, when the difference between the thermal expansion coefficients of the target and backing plate metals is great, the warpage and differential part contraction problems referred to above are presented.
Accordingly, there remains a need for a soldering method that is capable of providing a durable bond that may be applied to the requisite sputter target/backing plate interface at low temperatures.
There remains an even more specific need for the provision of a soldering method that may be employed to solder bond brittle ceramic target materials such as indium tin oxide materials including tubular indium tin oxide targets to a backing plate and titanium or titanium alloy materials to aluminum or copper backing plates.